618 A. G. BOURNE. 



The spermatozoa have been stated in some cases to arise 

 from the ectoderm while the ova arise from the endoderm. E. 

 von Beneden has shown this to be the case in Hydractinia, 

 Fraipont in Campanularia, and Weissmann in Gonothy- 

 rsea. These various modes of origin have been described as 

 existing in the same family. 



Weissmann's recent observations advance our knowledge a 

 considerable step in a slightly different direction ; he has 

 shown that there are a large number of species, of genera, and 

 even of families, in which the generative products do not 

 originate in reproductive individuals — gonophores — but in the 

 parenchyma of the trophosome, the coenenchyrae of Milne 

 Edwards and Haime, the coenosarc of Allmann, and that they 

 afterwards migrate to a " maturing bud " (gonangium). Such 

 an origin Weissmann terms coenosarcal, in contradistinction 

 to ablastoidal origin; and he would recognise two types of 

 Hydroids — coenogenous (coenosarcogenous), and blas- 

 togenous. 



The whole process especially with regard to the formation of 

 the gonangium and the migration of the sexual elements 

 into it is a remarkable one, and we may cite Weissniann^s 

 observations upon Plumularia echinulata (Lam.). 



Both varieties of sexual cells form in the endoderm of 

 the coenosarc, usually in the trunk of the colony, often 

 at the base of the lateral branches. At the time that the 

 sexual cells appear, there is no trace of gonangia. These 

 form in the coenosarc in the neighbourhood of sexual cells. 

 The sexual cells arise in a similar manner in both sexes, ova 

 and spermatospores (sperm mother-cells) arising by metamor- 

 phosis of ordinary endoderm cells : this Weissmann has observed 

 with great certainty. 



The gonangia develope with perfect regularity from below, 

 upwards, so that we can determine beforehand the precise spot 

 where a gonangium will later on develope. 



Where a gonangium is about to develope the first change 

 takes place in the ectoderm, which in the trunk consists of 

 several layers of cells. 



